Effect of wind characteristics on gas dispersion in porous media

Abstract

Greenhouse gases have the key role in global warming. Soil is a source of greenhouse gases such as methane (CH4). Radon (Rn) which is a radioactive gas can emit form soil into the buildings and causes health concerns. Different soil properties can affect gas emissions inside/from soil including temperature, humidity, air pressure and vegetation (Oertel et al., 2016). It’s shown in many cases that pressure fluctuations caused by wind play an important role in transport of gas in soil and other porous media. An example is: landfill gas emissions (Poulsen et al., 2001). We applied a novel experimental equipment for measuring controlled wind turbulence on gas transport in porous media. This set-up was utilized to evaluate the effect of wind turbulence on gas transport in relation to the depth of porous medium. Experiments were carried out with binary diffusion of CO2 and air as tracer gases with average vertical wind speeds of 0.02 to 1.06 m s-1. 13 different wind conditions with different speed and fluctuations were applied. Five oxygen sensors were places inside sample at different depths to measure air transportation within porous media and total of 39 experiments were carried out. Gas transport in porous media is described by advection-dispersion equation. Gas transport is quantified as a dispersion coefficient. Oxygen breakthrough curves as a function of distance to the surface of the porous medium exposed to wind were derived numerically with an explicit forward time, central space finite-difference based model to assess gas transport. We showed that wind turbulence-induced dispersion of gas is an important transport mechanism that can increase gas transport with average of 45 times more than molecular diffusion under no-wind condition. Power spectrum density is calculated for all the 12 wind conditions to determine strength vibration of all the wind speeds.